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3.3.8
Thermal design
The failure rate of semiconductor devices is greatly increased as operating temperatures increase.
As shown in Figure 2, the internal thermal stress on a device is the sum of the ambient
temperature and the temperature rise due to power dissipation in the device. Therefore, to
achieve optimum reliability, observe the following precautions concerning thermal design:
(1) Keep the ambient temperature (Ta) as low as possible.
(2) If the device's dynamic power dissipation is relatively large, select the most appropriate
circuit board material, and consider the use of heat sinks or of forced air cooling. Such
measures will help lower the thermal resistance of the package.
(3) Derate the device's absolute maximum ratings to minimize thermal stress from power
dissipation.
θja = θjc + θca
θja = (Tj–Ta) / P
θjc = (Tj–Tc) / P
θca = (Tc–Ta) / P
in which θja = thermal resistance between junction and surrounding air (°C/W)
θjc = thermal resistance between junction and package surface, or internal thermal
θca = thermal resistance between package surface and surrounding air, or external
Tj = junction temperature or chip temperature (°C)
Tc = package surface temperature or case temperature (°C)
Ta = ambient temperature (°C)
P = power dissipation (W)
3.3.9
Interfacing
When connecting inputs and outputs between devices, make sure input voltage (V
output voltage (V
connecting devices operating at different supply voltages, such as in a dual-power-supply system,
be aware that erroneous power-on and power-off sequences can result in device breakdown. For
details of how to interface particular devices, consult the relevant technical datasheets and
databooks. If you have any questions or doubts about interfacing, contact your nearest Toshiba
office or distributor.
3 General Safety Precautions and Usage Considerations
resistance (°C/W)
thermal resistance (°C/W)
Figure 2 Thermal resistance of package
/V
) levels are matched. Otherwise, the devices may malfunction. When
OL
OH
Ta
θca
Tc
θjc
Tj
3-8
/V
) and
IL
IH
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